Dual-string dynamometer for measuring dental handpiece power at high speed and low torque

ABSTRACT

A dual-string tension dynamometer ( 1 ) utilizes two strings ( 10, 11 ) wrapped around approximately ninety degrees of a test wheel ( 12 ). The lateral forces (E, F) are balanced such that the total lateral force net value is zero. The results reflect purely torsional loading.

RELATED APPLICATIONS

This application is a Continuation of U.S. Ser. No. 10/689,297 filedOct. 20, 2003, which claims the benefit of U.S. Provisional ApplicationSer. No. 60/419,374 filed on Oct. 18, 2002.

TECHNICAL FIELD

The present invention is generally related to string tensiondynamometers used to measure power in a high speed, low torque dentalhandpiece. More particularly, the invention relates to such adynamometer employing dual or two strings in order to avoid a lateralforce being exerted on the test wheel. The lateral forces are balancedresulting in a total lateral force net value of zero.

BACKGROUND OF THE INVENTION

The power output of high-speed, low torque dental handpieces, such asair turbine handpieces, can be measured using a string tensiondynamometer. For example, it is know to use a Kerfoot string tensiondynamometer, which is a device that applies a load to a handpiecethrough a string looped around a pulley mounted in a handpiece chuck(See FIG. 1). String tension is measured by the deflection of weighteddials to which the ends of the string are attached. Under steady-stateconditions, the net tension on the string multiplied by the pulleyradius is equal to the handpiece torque.

According to one test protocol, the face of the pulley is half blackedout for speed detection using an optical tachometer. The pulley shaftconforms to DIN 13950 and ISO 1797 (0.0628″, 1.595 millimetersdiameter). The pulley wheel is lightweight aluminum, unconcentricity notmore the 0.0003 inches. Each pulley is tested for balance at speeds upto 500,000 RPM by recording the free spin RPM of a new handpiece witheach pulley and discarded if they are statistical outliers. The maximumpower output of a handpiece occurs at a speed that is about half theno-load (or free-running) speed. To determine the power output, themaximum speed and the torque at half the maximum speed is measured.Torque and power can be measured as follows:P=vT, where v=RPM (2π)/60 and T=(T _(R)-T _(L))mgR

P is the power in Watts.

v is the speed expressed as angular velocity, radians per second.

RPM is the speed in revolutions per minute at which the torque wasmeasured.

π is the constant 3.14159.

T is the torque expressed as Newton-meters.

TR and TL are the right and left dial deflection readings (See FIG. 1).

m is the mass of the dial weights expressed in kilograms.

g is the gravitational acceleration, about 9.8 meters per second².

R is the pulley radius in meters (such as for example, 0.100″ or2.54×10⁻³ meters). The actual pulley radius is adjusted to compensatefor the thickness of the string. The effective pulley radius includingthe radius of the string is 0.100″.

While such dynamometers have proven valuable in determining power, theydo result in a small lateral force being exerted on the test wheel.Therefore, a purely torsional load never exists resulting in increasedmeasurement error. A need exists therefore, for a string tensiondynamometer which will avoid the torsional load-induced errors.

SUMMARY OF THE INVENTION

A dual-string tension dynamometer according to the present inventionutilizes two strings. The lateral forces are balanced such that thetotal lateral force net value is zero. The results reflect purelytorsional loading.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a Prior Art single stringtension dynamometer, showing the left and right dial deflection readingsas T_(L) and T_(R) respectively. The force vector representing tensionat T_(S) is equal to the sum of T_(L) and T_(R).

FIG. 2 is a schematic representation of a dual-string tensiondynamometer according to the present invention.

PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION

A single-string dynamometer works on the premise that the tensiondifference between two sides of a load string equals the force appliedto a test wheel at that specific radius. By summing vectors a resultantvector is found. This resultant cannot be zero due to the nature of thedynamometers operation. Additionally, this vector may include anorthogonal component if the load string is not mounted tangent to thetest wheel. According to the present invention, adding a second stringintroduces a second set of forces. By keeping appropriate tensionmagnitudes equal, a zero net force results. A state of purely torsionalloading has been reached.

A dual-string dynamometer 1 according to the invention includes twofilaments 10 and 11, wrapped around approximately ninety degrees of atest wheel 12. A conventional control device (not shown) may beintroduced to maintain equal tensions and a zero lateral load. Powervalues are determined by multiplying torque values with angular speeddata. Such relationships are expressed according to the followingequations, where A, B, C, D, E and F are force vectors at indicatedpoints of the filaments 10 and 11 as shown on FIG. 2.E=A+CF=B+DE+F=0

A is the tension on one end of string 10; B is the tension on a sameside of string 11; C is the tension on the other side of string 10 fromA; D is the tension on the other side of string 11 from B; as is shownrepresentationally in FIG. 2.

A dual-string dynamometer 1 as described eliminates lateral loading,which provides loading condition certainty. High speed, low torque powerdata can be accurately attained in an otherwise conventional manner.

It will be appreciated that according to the present invention, anyarrangement using two strings could be used. String wrap angle, wheelsize, and string material can be altered in order to accommodate testingsituations. Any means of controlling string tension could be used withvarying degrees of accuracy. Further, while such a device is especiallyuseful in testing dental high speed, low torque handpieces, such adevice has application to any number of applications. It has beendescribed herein with respect to the testing of dental handpieces onlyfor exemplary purposes and should not necessarily be so limited. Theinvention is limited only by the scope of the attached claims.

1. A string tension dynamometer comprising a test wheel, a firstfilament wrapped around said test wheel and a second filament wrappedaround said test wheel; wherein said first and said second filaments arewrapped around said test wheel on substantially opposite sides thereof;wherein lateral forces on said first and said second filaments aresubstantially balanced such that the total lateral force net value issubstantially zero.
 2. A string dynamometer as in claim 1, wherein saidfirst filament is wrapped around said test wheel to form about a 90degree angle.
 3. A string dynamometer as in claim 2, wherein said secondfilament is wrapped around said test wheel to form about a 90 degreeangle.